1. Field of the Invention
The present invention relates to the precision measurement of the positions, dimensions, etc., of a pattern or mark formed on the surface of a flat object, e.g., a photographic mask or reticle and more particularly to the measurement of the accurate coordinate positions of such pattern or mark which have been corrected for measurement errors due to a deflection of the flat object.
2. Description of the Prior Art
In the measurement of the coordinate positions of a microscopic pattern or mark formed on the surface of a flat object such as a photographic mask or reticle, any planewise distortion in the surface of the object has a considerable effect on the measuring accuracy. Where the measurement is effected by horizontally mounting the object on a holder having a plurality of supports arranged in correspondence to the sides of the object and vacuum chucking the object onto the supports, even if the supports are each provided on a horizontal plane having a good flatness and moreover all the supports are provided on the same horizontal plane, the central portion of the object is deflected downward by the effect of the gravity on the ground that the sides of the object are supported. Noting the vertical section of the object supported in this manner, the deflection thereat is caused in the like manner as the model of a simple beam supported at its ends. In this case, while no planewise run-out is caused at the neutral plane between the upper and lower surfaces of the object irrespective of the deflection according to the Bernoulli-Euler supposition in the strength of materials, the upper surface of the object or the surface containing the pattern or mark is reduced in the plane direction due to the occurrence of the deflection. Therefore, the above-mentioned construction of the holder involves an error in the result of measurement and it is thus not desirable.
Thus, a method has been used in the past in which the holder is processed to produce a plane surface on its whole supporting surface so that the whole area of the lower surface of the object is uniformly contacted with the supporting surface and air is sucked through slots or holes partically formed in the supporting surface thereby chucking the object onto the supporting surface. However, it is extremely difficult to finish the supporting surface to have a sufficient flatness and also it is difficult to force the object to follow the plane shape of the supporting surface by vacuum chucking. Thus, even with this method, the central portion of the object tends to project slightly with respect to the adjoining portion.
Another disadvantage is that if dust or the like deposits on the supporting surface, the lower surface of the object is forced upward at the position of the dust and the flatness is deteriorated.